The present invention relates, in general, to a method for removing nitrogen oxides (hereinafter referred to as xe2x80x9cNOxxe2x80x9d) using natural manganese ores and, more particularly, to a method for removing nitrogen oxides by a selective reduction catalytic technique using ammonia as a reductant and natural manganese ores as a catalyst, which shows excellent catalytic activity in reducing Nox contained in exhaust gas at low temperature, without further subjecting the ores to difficult and costly processing.
Many techniques have been suggested to remove the NOx contained in the exhaust gas from a source, such as a burner, a boiler, etc. Of them, selective catalytic reduction (SCR) techniques are now evaluated to be the most preferable in economic and technical aspects and extensive studies are being made on the topic of technique. In such an SRC technique, NOx, such as nitrogen monoxide (NO) and nitrogen dioxide (NO2), is reduced to nitrogen and water in the presence of a catalyst with ammonia serving as a reducing agent, as seen in the following reaction formulas I to IV:
6NO+4NH3xe2x86x925N2+6H2Oxe2x80x83xe2x80x83(I)
4NO+4NH3+O2xe2x86x924N2+6H2Oxe2x80x83xe2x80x83(II)
6NO2+8NH3xe2x86x927N2+12H2Oxe2x80x83xe2x80x83(III)
2NO2+4NH3+O2xe2x86x923N2+6H2Oxe2x80x83xe2x80x83(IV)
Whether the SCR techniques are successfully performed or not is dependent on the catalyst.
The catalysts used in the SCR technique have a common feature of being higher in the conversion rate of NOx as the reaction temperature increases. At high temperatures, however, ammonia is apt to be oxidized by reaction with the oxygen contained in the exhaust gas, to lose its function as a reductant, as shown in the following reaction formulas V and VI:
4NH3+5O2xe2x86x924NO+6H2Oxe2x80x83xe2x80x83(V)
4NH3+3O2xe2x86x922N2+6H2Oxe2x80x83xe2x80x83(VI)
Thus, the temperatures at which the conversion rate of the Nox reaches the maximum are different from catalyst to catalyst. In the current SCR technique, the catalyst in which V2O5 is deposited on TiO2 support is commercially used. Furthermore, in order to increase the activity of the catalyst and prevent poisoning by sulfur dioxide, W, Mo and the like are added. Such catalysts show an excellent NOx-removal performance, however have a problem that the optimum operational temperature is high, i.e., around 350xc2x0 C. A large amount of dust, pollutants and the like are included in exhaust gas at such temperature that the possibility of deactivation and poisoning of the catalyst increases. Accordingly, a dust removing apparatus is required. In particular, when the catalyst is installed in the back of a desulfurizing apparatus, additional heat for activation of the catalyst is needed. Under these circumstances, there has been a need to develop a catalyst which shows an excellent NOx-removing efficiency at low temperature ranges below 250xc2x0 C. Further, there is a need to develop a process for selectively reducing the catalyst at a low temperature in various field including the back of the heat recovery means of generator, co-generation system, shaft furnace, waste incineration and the like.
In general, among the catalysts suitable for selective catalytic reduction reactions at low temperature, manganese-based metal oxide catalysts are known to have a highest NOx-removing efficiency. The method of using natural manganese ores containing manganese oxides in a large amount has also been suggested.
In this regard, Japanese Patent Laid-open No. Hei 7-88334 discloses a process for removing nitrogen oxides at 90-150xc2x0 C. by using manganese mineral ores as a catalyst, but the main component of the manganese ores used in this process is xcex1-MnO2, which shows Nox-removal efficiency of below 70%, and thus cannot obtain a NOx-removal efficiency of more than 90%
U.S. Pat. No. 4,883,647 discloses a process for removing at least one of the pollutants from an exhaust gas by using manganese nodules. The above manganese nodules include mainly Fe, Mn, Si, Ca and P, the constituents of which are similar to natural manganese ores. However, the main component has a Mn content of 15-30 wt. % and is not present in oxide, but in crystalline phase. It contains a considerable amount of Pt, Ni, Co, Cu, Ti, Pb and the like, which are greatly different from natural manganese ores. Further, the above manganese nodule and the natural manganese ores differ greatly in view of natural morphology, production area, manganese content and physical property. The chemical composition and physical properties of manganese nodules are given as shown in Table 1, below.
The above patent discloses that nitrogen oxides can be removed by using manganese nodules as a catalyst and supplying ammonia as a reductant. However, according to the above patent, a NOx-removal efficiency of the catalyst ranges in 30-50% at temperatures of 250-350xc2x0 C.; in other words, the treatment temperature is high and the efficiency is low.
In the meantime, U.S. Pat. No. 3,975,498 discloses a process for absorbing and removing nitrogen oxides through a column in which an electrolytic manganese dioxide is packed and a process for regenerating the above column.
Further, U.S. Pat. No. 5,589,147 discloses a process for removing nitrogen oxides from exhaust gas in which a selective catalytic reduction technique using ammonia as a reducing agent is carried out in the presence of the catalyst containing metallic oxides such as Cu, Zn, V, Cr, Mn, Co, Fe, Ni, Pd, Pt, Mo, W, Ce and the like on a zeolite-based support such as ZSM-5, ZSM-11 and the like. Among the metallic oxides used in the above patent, manganese oxides include MnO, Mn3O4, MnO3, MnO2, Mn2O7 and the like. It is disclosed that they can be used in a single or a mixture form. However, the temperature of removing nitrogen oxides ranges from 200 to 1000xc2x0 C. As shown in Examples 3 and 5, the conversion rate is low at 250xc2x0 C., the conversion rate increases as the temperature increases, and it exhibits the conversion rate of more than 70% at 550xc2x0 C. In this light, the above catalyst is difficult to be regarded as a NOx-removal catalyst employable at a low temperature. Accordingly, there is a need to develop a catalyst which can remove nitrogen oxides at a high efficiency even at a low temperature of below 250xc2x0 C., and which can be prepared by a more convenient method.
In light of the above problems, the present inventors have conducted intensive and through researches on the selective removal of the NOx contained in exhaust gas and, as a result, discovered that when natural manganese ores containing pyrolusite xcex2-MnO2 as a main component is used as a catalyst for selective catalytic reduction, the prepared catalyst shows excellent catalytic activity in reducing NOx at low temperatures (130-250xc2x0 C.), without further subjecting the ores to difficult and costly processing.
It is therefore an object of the present invention to overcome the above problems encountered in prior arts and to provide a method for removing NOx contained in exhaust gas, by which the NOx contained in exhaust gas is reduced at relatively low temperatures at an excellent efficiency.
It is another object of the present invention to provide a method for preparing a catalyst for removing NOx contained in exhaust gases, which is relatively simple and economically favorable.
In accordance with the present invention, the above objects could be accomplished by a provision of a method for removing the nitrogen oxides in exhaust gas, characterized in that a selective catalytic reduction technique using ammonia as a reductant is carried out at a temperature of 130-250xc2x0 C. in the presence of a catalyst prepared from natural manganese ores to remove the nitrogen oxides, wherein the natural manganese ores contain 50-90 wt % of pyrolusite of xcex2-MnO2 as a main component.